This invention relates generally to microwave devices and more particularly to nonreciprocal microwave devices such as circulators and isolators.
As is known in the art, energy transfer between two or more ports may be provided by non-reciprocal devices such as circulators and isolators. In particular with circulators, energy fed to an input port of the circulator is transferred to an output port of the circulator whereas input energy fed to the output port of the circulator is not efficiently transferred to the input port of the circulator and, therefore, such a device is generally referred to as a non-reciprocal device. One type of circulator commonly employed in the art is a so-called junction circulator which can be comprised of either a stripline transmission medium or microstrip transmission medium, for example. Commonly, such junction circulators are provided with three ports and are generally referred to in the art as Y-junction circulators. The basic construction of a stripline Y-junction type circulator includes a center patterned conductor having three stripline branches connected to a central portion. The center conductor is sandwiched between a pair of dielectrics which dielectrically space said central conductor from a pair of ground planes disposed over second surfaces of the pair of dielectrics. Disposed in said dielectrics is a pair of discs comprised of a ferromagnetic material which exhibits gyromagnetic action. Commonly, materials such as ferrites and garnets are used to provide the gyromagnetic action. The ferromagnetic materials are disposed within the presence of a DC magnetic field. Input energy fed to an input one of the branches of the circulator is transferred either to a clockwise disposed or counter-clockwise disposed adjacent output one of the ports of the circulator in accordance with the polarity of the DC magnetic field fed through the ferrite disc members.
Generally, circulators are relatively narrow band devices capable of operating with acceptably low insertion loss and relatively high isolation over about an octave of bandwidth. A device having an octave bandwidth has an upper frequency limit which is equal to twice the lower frequency limit of the device. Accordingly, a circulator which operates at a low frequency of 2 GHz having an octave bandwidt would have an upper frequency limit of 4 GHZ.
Two types of circulations are known in the art. The first type, the so-called "edge mode" circulator is arranged to have the energy traveling through the ferrite medium concentrated near one of the edges of the center patterned conductor. Circulator action is provided at the junction. As the energy propagates towards the junction, it will propagate towards the port which is closest to the edge at which the energy was concentrated. The second type of circulator operates in a mode as described by Wu and Rosenbaum in an article entitled "Wideband Operation of Microstrip Circulators MTT 22, Pages 849-856, October 1974. Here, the ferrite members operate as dielectric resonators.
Many attempts have been made in the prior art to increase the bandwidth of circulators. Two such attempts are described in U.S. Pat. No. 3,555,459 by Anderson for what is here considered circulator and U.S. Pat. No. 4,496,915 by Mathew et al for dielectric resonator mode circulators.
In the '459 patent broad-band performance was provided by attempting to reduce or supress various undesired modes at the upper end of the band by arranging the central conductor of a stripline type circulator to have smooth and tapered branch legs connected to a central portion. The tapered legs were free of abrupt changes in direction to assure that the rate of change of the edges of the conductor was less than the rate of circulation of a TEM mode wave in a transversally magnetized ferrite so that no large mismatches occurred which would otherwise increase the frequency sensitivity of the device. Patentee further describes the use of mode suppressing techniques which involved placing a lossy dielectric material adjacent the ferrite disc to suppress undesired higher order modes. This is an example of an "edge mode" propagation circulator. Broad band performance is achieved at the expense of size. That is, in order to concentrate energy at the edge of the stripline medium, relatively large diameter ferrite discs (i.e. 1.0 in. to 3.0 in.) are required.
A solution to increasing the bandwidth of microwave circulators, which operate on the principal described by Wu and Rosenbaum, is described in the '915 patent to Mathew. Patentee uses a composite ferrite disc between the central conductor and ground planes of the circulator circuit. The composite ferrite disc includes two concentric members, with each member comprised of a different ferrite material and with one member being disposed within the periphery of the other member. The two materials are selected to provide different frequency characteristics over the frequency passband of the circulator. This approach requires relatively difficult fabrication techniques to provide the composite ferrite, which may increase the cost of such circulators. Further, this approach also increases the diameter of the ferrite discs, thus making the circulator larger.
In each of these references as well as the art in general, the lower limit on the bandwidth of circulator operation is recognized to be at a frequency known as the magnetization frequency f.sub.m, which is given by 2.gamma.f.sub.m =.pi.4.gamma.M.sub.s where .gamma. is is the gyromagnetic ratio of the ferrite and 4.pi.M.sub.s is the saturation magnetization of the ferrite. Available ferrite materials have f.sub.m values from a fraction of a GHz up to approximately 14 GHz. The bandwidth of conventional circulators is limited on the low end at f.sub.m by the onset of a phenomena known as "low field loss".
At the high end of the frequency bandwidth, the frequency band of circulators is limited by the fact that the gyromagnetic effects of ferrite material generally become small as the frequency is increased.
With the approaches discussed above, several problems exist particularly for applications which require small, compact circulators. Patent '459 achieves improved broadband performance by mode suppression, rate of circulation matching, and use of relatively large ferrite discs which provide an edge mode propagation type of circulator. Thus, while the bandwidth of the '459 device is shifted towards f.sub.M, it comes at the expense of requiring the use of a very large ferrite disc. Patent '915 achieves improved broad-band performance by using a composite ferrite disc located within the magnetic circuit of the circulator. With this arrangement, construction of the circulator becomes more difficult. Further, the composite disc also increases the size of the circulator. For certain applications such as in a transceiver of a phased array antenna, circulator size is extremely important since spacing of antenna elements on the face of the phased array is related to the wavelength of the energy being transmitted and received.